served the formation of iodobiphenyls when alarge excess of iodobenzene is used (30).

Low-temperature (77 K) electron paramagnetic resonance (EPR) data are consistent with
photoinduced generation of a copper-containing
radical when copper–carbazolide complex 1 and
iodobenzene are irradiated with a 100-W mercury lamp at –40°C for 15 min in a 4:5 mixture of
propionitrile/butyronitrile (Fig. 3A); this signal
and the deep-blue color of the reaction mixture
rapidly disappear upon warming to room temperature. The EPR spectrum has nearly axial symmetry. Coupling to 63/65Cu is too small relative to
the broadness of the signal to be observed (31);
nevertheless, the strongly anisotropic g-values suggest that this species has at least partial metallo-radical [Cu(II)] character. The same EPR spectrum
is produced when complex 1 is treated at –100°C
with 0.3 equivalents of the oxidant Magic Blue
[tris(4-bromophenyl)aminium hexachloridoanti-monate], indicating that the same radical species
can be generated by chemical and by photoinduced
oxidation. We speculate that rather than being
radical cation 3 itself (Fig. 1C), the detected radical is likely a more stable derivative, such as
Cu2[P(m-tol)3]4(carbazolide)2+•, which could be
formed via trapping of radical cation 3 by complex 1. The lack of resolved 63/65Cu hyperfine
coupling in the EPR signal is consistent with such
a species (32), as is our observation that this signal is absent when one equivalent of Magic Blue
is used.

To provide further support for radical intermediates in these photoinduced Ullmann reactions,
we examined the coupling of copper–carbazolide
complex 1 with 2-(allyloxy)iodobenzene (4), a
radical probe (Fig. 3B). Because radical 5 is
known to cyclize very rapidly [rate constant (k) =
9.6 × 109 s−1 in dimethyl sulfoxide] (33), the consistent failure to observe cyclized products in other
studies of Ullmann C–N bond-forming reactions
with this substrate has been cited as evidence
against a radical pathway (12–15). In contrast, in
the case of our photoinduced Ullmann coupling
we observed exclusive formation of cyclized compounds (6 and 3-methyl-2,3-dihydrobenzofuran)
(Fig. 3B). This dichotomy of reaction products
under thermal versus photochemical conditions
does not require a divergence in pathways (
radical versus nonradical) for C–X cleavage; for example, the thermal process could, in principle,
proceed through an aryl radical intermediate that
is captured by copper faster than it adds to the
pendant olefin.

Control reactions establish that copper complex 1 does not couple with 4 in the dark, even
upon heating to 65°C. Furthermore, no cyclized
products are detected when compound 4 is irradiated in the absence of complex 1.

Formation of dihydrobenzofuran 6 does not
conclusively support a radical pathway because
this product could in principle be generated via
concerted oxidative addition of 4 to form an aryl–
copper reagent, followed by b-migratory insertion
and reductive elimination. However, our observation